When I switch from single to image sequence of an openEXR image sequence in the Compositor Panel, the backdrop disappears
SOLVED: My OpenEXR multilayer has 10001,10002,10003... as suffix numbers. This means that the "offset" ot the Image Node must be set as 1000, to reach the frame 1,2,3 etc... Only importing the OpenEXR from the node folder, selecting all the images and clicking import, has forced Blender to automatically set the offset at 1000, resolving this issue. Of course this could be done manually.